Radio frequency and microwave circuits include active or power driven components, such as amplifiers and the like, which form an operational path for signals passing between the input and output terminals of the path. Circumstances sometime exist when it is critical from a reliability standpoint, for these signals to be always capable of passing between the input and output terminals. As such, it is often necessary to provide a bypass path around these components in the event of a component disablement due to a loss of supply power. The bypass path routes around the disabled component while maintaining continuous, but degraded system performance. Such a circumstance would involve signals propagating in a communication or data application. In these cases, disablement would involve a loss of communication as well as data. Prior art bypass circuits typically employ two electromechanical relays, and RF transmission line configuration. During normal operation, system operating voltage is applied to the coil windings of the relays which hold the contacts in a position which causes the signal to pass through the component. In the event of a power loss, the relays release the contacts, and the signal is then routed around the disabled component via a section of transmission line. These arrangements operate over a very wide bandwidth, and contribute only a small loss in signal level.
Prior art bypass circuits, however, exhibit several problems related to reliability, size, and power consumption. In the operating mode, the relay coils must be energized which consumes a significant amount of electrical power. This reduces the efficiency of the system and requires a larger power supply. Furthermore, additional heat is generated by the relays which must be dissipated. Reliability of the .circuit suffers because the contact system of the relays is typically required to make and/or break millions of times and provide a low, stable electrical resistance over the lifetime of the relays. Because of the numerous operations and arcing often occurring during operation, the contacts are subject to a wide variety of hazards that may cause failure. Moreover, electromechanical relays have other disadvantages including their physical size; they can generate radio frequency interference (RFI) by switching of their coils, thereby requiring special shielding to isolate the RFI; their switching speed is slower compared to solid state components of like function; and generally they occupy more space than the component being bypassed.
The present invention solves the problems of prior art bypass circuits by using all solid state components including field-effect-transistors (FETs) and PIN diodes. Solid state components operate at very low power levels. For this reason, utilization of solid state components in bypass circuits insures high reliability, low power dissipation, and very small size.
A FET is a voltage-controlled semiconductor device in which the current conduction between the source and the drain regions is controlled or modulated by means of a control voltage applied to the gate electrode. For an example of FETs used as switching devices at microwave frequencies, see U.S. Pat. No. 5,093,667 issued on Mar. 3, 1992 entitled "T/R MODULE WITH ERROR CORRECTION" by C. Andricos and assigned to ITT Corporation.
A PIN diode is a semiconductor device used as a switch or attenuator in microwave transmission lines. PIN diodes include an undoped high-resistivity region between p and n regions. This central region is typically a lightly doped p-layer or lightly doped n-layer. Both FET's and PIN diodes are available for operation in the microwave range.
During normal operation, the circuit according to the present invention, consumes approximately 20 milliwatts of dc power, compared to several watts for prior art bypass circuits which utilize electromechanical relays. In the bypass mode, the present invention uses no power. Solid state components generally include no moving parts and as such can be cycled on and off nearly an infinite number of cycles.
It is, therefore, an object of the present invention to provide a bypass circuit for bypassing a signal around disabled component located in a critical signal path, which utilizes all solid state components to reduce the power consumption of the bypass circuit, insures high reliability of the bypass circuit, and reduces the size of the bypass circuit.